I manually disabled MTRRs in the kernel (in /sys/i386/i386/i686_mem.c) , XFree86 (config/cf/FreeBSD.def and host.def) and added the 'Option "NoMTRR"' line to the Screen section in /etc/X11/XF86Config (shouldn't make a difference as I disabled MTRRs at compiletime).
sysctl -a revealed the gart driver (either FreeBSD or nVidia) was never enabled (througout all my tests and configurations, also has to do with problems with MTRRs, I read).

The X server starts and works great, until I run OpenGL programs. The system would just hang and reboot. After trying lots of different combinations in the BIOS and enabling/diabling MTRRs in all parts (X, kernel) no progress was made.

My testprogram is glxinfo, and I "stresstest" my configuration by running it multiple times. Sometime this evening I saw a simple pattern:

* when logged in as root on the console and having started X with startx, I had no problem running glxinfo tens of times after each other
* when logged in as non-root on the console and having started X with startx, glxinfo would crash after just 3 to 4 times of running it. Running it yet again, would hang and crash the system most of the time.

My X environments in root and non-root were equal, no .xsession so X starts with a single window without windowmanager.

Has anyone seen this before and/or has a solution?

I can provide a coredump of glxinfo if nessecary, but haven't tried to receive the kernel dumpmessages on my serial port.